With the latest remarkable developments in lightening, thinning or miniaturization of electric and electronic instruments, not only lightening, thinning or miniaturization of various conductive materials used therein but also new development of these materials per se have been desired.
Various sulfur-containing heterocyclic polymers are known including polymers from thiophene, U.S. Pat. No. 2,552,796 and U.S. Pat. No. 2,658,902; polymers from dibenzothiophene, U.S. Pat. No. 3,585,163; polymers from vinyl bithiophene, U.S. Pat. No. 3,615,384; polymers from various substituted thiophenes, U.S. Pat. No. 3,725,362; polymers from 2-bromo-8-hydroxy-5,5-dioxodibenzothiophene, U.S. Pat. No. 3,775,368; and polymers from tetrathiapentalene, U.S. Pat. No. 4,111,857.
Within the rapidly expanding field of polymeric conductors ("Proceedings of the International Conference on the Physics and Chemistry of Polymeric Conductors", J. Physique. Colloque, 1983, C-3), the poly(heterocycles) have received attention because they are easily prepared in film form and are considerably more stable to atmospheric exposure than poly(acetylene) or poly(phenylene). For use in stabilizing a semiconductor surface, see R. Noufi et al., J. Amer. Chem. Soc., 1981, Vol. 183, 184 and references therein. A further extension of this work is our recent entry into the study of poly(thiophene).
Extensive investigations on new conductive high polymers have been conducted. For example, polyacetylenes are under investigation for their possible availability as electrode materials of secondary batteries since they show excellent conductivities as high as 10.sup.2 to 10.sup.3 s/cm by doping iodine or arsenic pentafluoride (cf. Synthetic Metals, Vol. 1, No. 2, 101 (1979/1980)), and excellent charge-discharge characteristics. Further, use of polyacetylenes as materials for solar batteries is also under investigation because of their light absorption characteristics close to those of sun light. However, the polyacetylenes are disadvantageous in that they are per se susceptible to oxidation and doped polyacetylenes are extremely sensitive to humidity.
Polythiophenes are studied not only as conductive materials or as battery electrode materials because of their specific electronic structure having a conjugated structure similar to that of cis-form polyacetylenes and containing a sulfur atom, but also as electrochromic materials making use of color changes in a doped state. For example, A. M. Druy, et al reported that 2,2'-bithienyl is electrochemically polymerized to form a polymer having a color which reversibly varies from blue in an oxidized state to red in a reduced state and such a polymer is useful as an electrochromic material making use of the reversibility of the color change [cf. Journal de Physique, Vol. 44, No. 6, C3-595 (1983)].
In the light of the above-described problems, the present inventors have conducted extensive investigations and, as a result, found that a polymer having an isothianaphthene structure is a very stable compound even in air and capable of reversibly varying its color in the course of oxidation or reduction in such a stable manner as sufficient to allow repeated use thereof and it is a novel polymer that can easily show conductivities higher than 10.sup.-2 s/cm upon doping general dopants and, thus accomplished the present invention.
According to the present invention, we have now synthesized poly(isothianaphthene), a polymer of a "nonclassical" thiophene (M. P. Cava et al., Acc. Chem. Res., 1975, Vol. 8, 139). While not bound by any theory, we believe that poly(isothianaphthene) exhibits higher stability (and perhaps conductivity) than poly(thiophene) because the resonance contributors 1c and 1d shown in FIG. A are important in the stabilization of open shell species (1c) and delocalization along the backbone (1d) is responsible for high electrical conductivity. ##STR2##
The analogous resonance structure (particularly the analog of ld) would not be expected to be as important contributors to the electronic structure of poly(thiophene) as they are in the case of poly(isothianaphthene) because of the overwhelming gain in stability resulting from incorporation of the 3,4 bond of thiophene into a benzene ring.
In the case of the preparation of poly(thiophene), the two simplest methods are anodic polymerization of pure thiophene (A. Diaz, Chem. Scripta, 1981, Vol. 17, 145; G. Tourillon et al., J. Electroanal. Chem., 1982, Vol. 135, 173; C. Kossmehl et al., Makromol. Chem. Rapid Commun., 1981, Vol. 2, 551; J. Bargon, IBM, J. of Res. and Dev., 1983, Vol. 27, 330; K. Kaneto et al., J Chem. Soc. Chem. Com., 1983, 382), and chemical coupling of 2,5-dihalothiophenes (M. Kobayashi et al., Synthetic Metals, 1984, Vol. 9, 77; T. Yamamoto et al., J. Polym. Sci., Polym. Lett., 1980, Vol. 18, 9; J. Lin et al., J. Polym. Sci., Polym. Chem. Edition, 1980, Vol. 18, 2869). The former procedure provides improved materials if 2,2-dithienyl is employed as starting material (M. A. Druy, J. Physique. Colloque, 1983, Vol. C-3, 595) and the electrolysis is carried out at relatively low applied voltages ( .about.3.5 V). From a practical point of view, the anodic polymerization is the more desirable; it is simple and the product appears in the form of a relatively tough, blue-black film. The chemically coupled product is of more academic interest since it is crystalline and its numberaverage molecular weight is known but it is invariably produced in powder form.
We have found that the most desirable approach to poly(isothianaphthene) is through the electrochemical coupling of isothianaphthene. (Monomer prepared according to J. A. Gadysz et al., Tetrahedron, 1979, Vol. 35, 2239; M. P. Cava et al., J. Amer. Chem. Soc., 1959, Vol. 81, 4266; M. P. Cava et al., J. Org. Chem., 1971, Vol. 36, 3932.
In this patent, we present procedures (electrochemical and chemical) for the preparation of poly(isothianaphthene). As will be shown below, electrochemical polymerization to yield the desired unsaturated polymer is possible only under rather specific conditions.
As is well known, liquid crystal display devices have recently been developed as display devices requiring low energy and been widely used in various applications. However, liquid crystal devices have a problem of dependence on a visual angle and also are disadvantageous in that the display is poor in sharpness; no memory function is provided; display cannot be obtained over a large surface area; and the like. In order to eliminate these disadvantages, studies have extensively been conducted on ECD devices of low energy type making use of the so-called electrochromism in which light absorption characteristics vary due to application of voltage or electric current. Electrochromic materials which can be used in the ECD devices are classified into inorganic materials and organic materials. The inorganic materials that are considered usable mainly include oxides of transition metals, a specific example is wolfram oxide, but they are limited in developable colors and also cause electrochemical elution of the membrane or deterioration of electrodes when protons are used as color-forming ions, although their response speeds are high. On the other hand, the organic materials include viologen dyes, phthalocyanine complexes, etc. However, the viologen dyes are disadvantageous in that repeated use thereof results in precipitation of insoluble substances, and the phthalocyanine complexes have a pending problem of adhesiveness between a vacuum-evaporated membrane and a base plate.
In addition, electrochromic materials which have recently been proposed include polyanilines as disclosed in A. F. Diaz, et al., Journal of Electro-Analytical Chemistry, Vol. 111, 111 (1980) or Yoneyama, et al., ibid., Vol. 161, 419 (1984); polypyrroles as disclosed in A. F. Diaz, et al., ibid., Vol. 101 (1983) and polythiophenes as disclosed in M. A. Druy, et al., Journal de Physique, Vol. 44, June, page C3-595 (1983) or Kaneto et al., Japan Journal of Applied Physics, Vol. 23, No. 7, page L412 (1983), but none of them has not yet been put into practical use. In particular, electrochromic materials are required to exhibit rapid response, provide high contrast, consume low power, develop excellent color tones and the like. Furthermore, an electrochromic material capable of developing a colorless tone will greatly contribute to broadening the utility of the device. However, any of these hetero-conjugated type high molecular weight materials are colored in the course of conversion from an oxidized state into a reduced state. Methods for increasing contrast by, for example, employing a white background plate, have been attacked but still not reached completion.